In general, the present invention relates to the removal of inorganic contaminants from groundwater. More particularly, this invention relates to a sandwich barrier for use with either a passive treatment system employing an array of non-pumping wells or in a passive treatment trenching system for removal of such contaminants.
As interest in environmental issues has increased, so has interest in improved methods of cleaning the land, air and water. In this regard, the interest in improved methods of groundwater cleanup has been acute. Potable ground-water supplies worldwide are contaminated by, or threatened by, advancing plumes containing radionuclides and metals. Unlike air, land and above groundwater cleanups, removal of contaminants from groundwater areas presents special problems with respect to gaining access to the contaminated areas which can make these cleanups very difficult.
Once a contaminant enters a groundwater system, it can be transported by the groundwater to a great depth. Additionally, such activities as mining or deep underground storage of radioactive or toxic materials can create a flow of contaminants through groundwater hundreds of feet below the surface. Contamination by uranium and other radioactive materials is particularly troubling because of the longevity of these materials.
Conventional methods for treating deep groundwater contamination are largely unsatisfactory because of cost and other considerations. Active methods that seek out the undergroundwater and bring it to the surface for treatment are most commonly used. The active methods most frequently used are so-called pump and treat methods, trenching methods, or some mixture of the two.
In a pumping method, contaminated groundwater is pumped from the contaminated lower regions to the upper regions where the groundwater may be treated and returned. The water may be brought up to the surface using drilled holes, trenches, or other means that provide the pumping equipment access to the water. Pump and treat systems suffer from several problems. The power requirements for a pump and treat operation are very high, and can be prohibitively expensive. This is particularly true of pump systems that treat deeper ( greater than 100 feet below land surface) groundwater. Further, these systems are costly and difficult to maintain.
A trenching system employs a trench dug into the ground down to the point of contamination. The exposed water is pumped from the trench or treated on site as the water flows through the trench area. Trenching systems work well in some instances of shallow contamination but run into difficulties when applied to contamination deeper within the ground because of the high cost of digging the trench, monitoring costs and issues of ground disturbance and recontouring. Another great disadvantages of trenching systems is the high cost of pumping the contaminated water from the trench to the surface for cleanup.
Because of the high costs of active systems using pump means, interest has increased in passive systems that can treat undergroundwater without a need for actual removal of the water. The use of arrays of non-pumped wells has been proposed as a method to remediate contaminated groundwater when the installation of treatment wells is not possible because of technical or financial constraints, for example, when contaminant streams are  greater than 200 feet below the land surface.
Prior art of interest include U.S. Pat. Nos. 5,512,702 (Methods for in-situ immobilization of lead in contaminated soils, wastes, and sediments using solid calcium phosphate materials); 5,514,279 (System for treating contaminated groundwater); and 5,362,394 (System for treating contaminated groundwater).
Important limitations of prior art treatment materials include the following: the materials are effective only with respect to a small number of contaminants, and the treatment materials are difficult to replace.
It will be evident from the foregoing that there is a need in this art of contaminated groundwater treatment for a passive, low cost materials for groundwater treatment that can treat a wide variety of underground contaminants including radioactive contaminants, can treat a large amount of water per unit, can be incorporated into a passive treatment system, and offers simple retrieval, replacement, and disposal of the reactive material used. In this regard, an alternative, cost effective approach to pump-and-treat methods could be widespread applicability to the treatment of contaminated groundwater associated with abandoned and active mine sites throughout the United States and other parts of the world.
In accordance with the invention, an improved permeable barrier is provided for decontaminating groundwater. The barrier can efficiently treat a wide variety of contaminants as part of a low-cost passive treatment system, and the barrier is simple and inexpensive to replace.
The barrier is disposed in a flow path for groundwater to be decontaminated and is comprised of a bone-char phosphate and iron oxide pellets.
In one embodiment, the phosphate and pellets of the barrier are a substantially uniform admixture.
In an alternative embodiment, the barrier comprises vertically alternating layers of the phosphate and pellets.
Preferably, the barrier is used for decontaminating a radionuclide from water. The radionulide decontaminated from water is preferably uranium.
In another aspect of the invention, a system is provided for decontaminating groundwater, the system comprising an array of non-pumping treatment wells disposed in a flow area for groundwater to be decontaminated, each of the wells containing a barrier comprised of bone-char phosphate and iron oxide pellets for decontaminating groundwater flowing therethrough.
In yet another aspect of the invention, a system is provided for decontaminating groundwater, the system comprising a trench disposed in a flow area for groundwater to be decontaminated and a barrier comprised of bone-char phosphate and iron oxide pellets disposed within the trench for decontaminating groundwater flowing therethrough.
In still another aspect of the invention, a groundwater decontamination system is provided, the system comprising a non-pumping well decontamination unit, a barrier comprised of bone-char phosphate and iron oxide pellets incorporated within the decontamination unit, and means for lowering the unit down through a non-pumping well into an acquifer containing contaminated water.
Further features and advantages of the present invention will be set forth in, or will be apparent from, the detailed description of a preferred thereof which follows.